The new copper(II) complex dichloridobis(4-benzoic acid)copper(II) methanol sesquisolvate hemihydrate, [CuCl2L2]·1.5CH3OH·0.5H2O, (1), has been synthesized from CuCl2·2H2O and the ligand 4-benzoic acid (L, C15H11N3O2). The complex was characterized by elemental analysis, Fourier transform IR spectroscopy, electrospray ionization mass spectrometry and single-crystal X-ray diffraction. Two chloride ligands and two bidentate L ligands coordinate the CuII centre in 1 in a Jahn–Teller-distorted octahedral geometry of rather unusual configuration: a chloride substituent and a pyrazole N atom of an N,N′-chelating ligand occupy the more distant axial positions. Classical O—H…O hydrogen bonds and O—H…Cl interactions link neighbouring complex molecules and cocrystallized methanol molecules into chains that propagate parallel to the b direction. The title compound shows intriguing bioactivity: the effects of 1 on the enzymatic activity of protein tyrosine phosphatase 1B (PTP1B) and on the viability of human breast cancer cells of cell line MCF7 were evaluated. Complex 1, with an IC50 value of 0.51 µM, can efficiently inhibit PTP1B activity. An enzyme kinetic assay suggests that 1 inhibits PTP1B in a noncompetitive manner. A fluorescence titration assay indicates that 1 has a strong affinity for PTP1B, with a binding constant of 4.39 × 106M−1. Complex 1 may also effectively decrease the viability of MCF7 cells in an extent comparable to that of cisplatin (IC50 = 6.3 µM). The new copper complex therefore represents a promising PTP1B inhibitor and an efficient antiproliferation reagent against MCF7 cells.